UV/Vis Spectroscopy of Copper Formate Clusters: Insight into Metal-Ligand Photochemistry.

Abstract

The electronic structure and photochemistry of copper formate clusters, CuI 2(HCO2)3 − and CuII n(HCO2)2n+1 −, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with CuI and CuII copper ions is observed. For the CuI species, transitions between copper d and s/p orbitals are recorded. For stoichiometric CuII formate clusters, the spectra are dominated by copper d–d transitions and charge‐transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low‐lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In CuII(HCO2)3 −, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge‐transfer excitations, specific excited‐state reaction channels are observed in addition, such as formyloxyl radical loss. In CuI 2(HCO2)3 −, the system relaxes to a local minimum on an excited‐state potential‐energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a CuII(HCO2)3Cu0− biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.